The present invention relates to an optical microscope having an objective lens interchanging mechanism.
When observing a specimen such as a vital cell, observation is performed while manipulating the specimen with a manipulator and measuring the potential with an electrode. In this case, the entire portion of the specimen is observed with a low-magnification objective lens to determine the portion to be observed. Thereafter, the low-magnification lens is switched to a high-magnification lens, and observation is performed in detail.
Conventionally, an optical microscope having an objective lens interchanging mechanism is proposed in order to use, by switching, objective lenses having different magnifications in this manner. For example, optical microscopes disclosed in Jpn. UM Appln. KOKAI Publication No. 6-40910 and No. 6-4720 and Jpn. Pat. Appln. KOKAI Publication No. 8-338940 are known. In each of these optical microscopes, a plurality of objective lenses having the same mount screw diameter that complies with the international standards (parfocal distance: 45 mm; mount screw diameter W: 20.32; and thread height: 36) and different magnifications are mounted to the support. An objective lens having an optimum magnification can be inserted on the observation optical axis in accordance with a change in observation magnification.
When the objective lenses are to be used in fluorescence observation by switching their magnification in this manner, as a low-magnification objective lens for observing the entire portion of the specimen, one having a magnification as low as possible is usually preferable. Conventionally, the lower limit of the magnification that can be employed is about lox. This is because the lower the magnification, the darker the observation image. To observe a fluorescent image which is originally dark, the magnification must be at least about 10.times.. If not, observation becomes impossible.
To allow fluorescence observation at a low magnification, the observation image must be made bright. The fluorescent intensity may be increased by increasing the intensity of excitation light. If the intensity of the excitation light is excessively increased, the specimen may be damaged or discolored with fluorescence, causing a trouble in observation.
To eliminate this, as a low-magnification objective lens, for example, one having a magnification lower than 10.times. and capable of ensuring a sufficiently high brightness for the observation image may be used. According to the optical microscope having the objective lens interchanging mechanism described above, the objective lenses to be mounted to the support must comply with the same standards and have the same mount screw diameter. If the objective lenses have different mount screw diameters, they cannot be mounted to the objective lens interchanging mechanism simultaneously and cannot be interchangeably used. If the objective lenses have different parfocal distances, when they are replaced, the focal point is largely displaced from the specimen surface due to the parfocal difference, and focusing must be performed again, leading to inconveniences in use.
For this reason, an optical microscope which can stably perform fluorescence observation with a low-magnification objective lens is sought for. Particularly, the following requirements are desired. A conventional illumination optical system must be used to suppress an increase in manufacturing cost, the compactness of the microscope must be maintained, and the microscope must be excellent in operability.
The objective lens switching operation described above poses the following problems.
Generally, in a microscope, a plurality of objective lenses are detachably held by an objective lens revolver which performs a switching operation among the plurality of objective lenses such that they can be inserted in and removed from the observation optical axis. Observation at a desired magnification is performed by turning the objective lens revolver to switch the objective lens on the observation optical axis.
In a microscope, since the height of eye point (the distance from the desktop surface to the operator's eye) with which the operator can perform observation with a natural posture is substantially fixed, the sizes of the respective portions of the microscope are limited. For example, the distance from the mounting surface of the objective lens, with which the objective lens is to be mounted to the objective lens revolver, to the sample surface (this distance will be referred to as the parfocal distance hereinafter) is usually designed to be about 45 mm. In an objective lens having a very low magnification of 1.times. or less, its parfocal distance is as very long as about 200 mm. If such an objective lens is used, its entire length cannot be accommodated within the parfocal distance. Therefore, it is impossible to change the observation magnification by only turning the objective lens revolver to switch the objective lens.
In order to solve this problem, conventionally, a microscope disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-54253 is known. According to this reference, one of a plurality of first objective lenses is defined as a very-low-magnification objective lens. The second objective lens fixed in an optical path is arranged with respect to a revolver means that performs a switching operation among the plurality of first objective lenses so that the selected one is located in the optical path. An observation image of an object is formed through the first objective lens selected by the revolver means and the second objective lens. A very-low-magnification auxiliary lens, which is to be inserted in an interlocked manner with selection of the very-low-magnification objective lens done by the revolver means, is arranged in the optical path between the first and second objective lenses. The first very-low-magnification objective lens is constituted by the very-low-magnification auxiliary lens and the very-low-magnification objective lens. More specifically, the very-low-magnification objective lens, the entire length of which cannot be accommodated within the parfocal distance, is divided into two portions, i.e., the first objective lens mounted to the revolver means, and the very-low-magnification auxiliary lens, and is constituted such that its synthetic focal distance becomes about 200 mm. The observation magnification, including the very low magnification, can be changed by inserting and removing the very-low-magnification auxiliary lens in and from the optical path in an interlocked manner with selection of the very-low-magnification objective lens by the revolver means.
In the microscopes disclosed in the above references, merely the very-low-magnification auxiliary lens is arranged in the optical path between the first and second objective lenses, and no description is made concerning the practical arrangement of the very-low-magnification auxiliary lens. For this reason,
(1) For example, when an extra space is newly prepared exclusively for the very-low-magnification auxiliary lens, not only the eye point described above becomes high, but also the entire microscope becomes large.
(2) The switching mechanism for inserting and removing the very-low-magnification auxiliary lens in and from the optical path must be prepared exclusively for the very-low-magnification objective lens, and must be interlocked with the turning operation of the revolver means. This leads to a complicated arrangement and an increase in cost, which is not preferable.
(3) Although the observation magnification can be changed, the microscopic method must be switched by separately providing a switching mechanism, resulting in a degradation in operability.
Generally, a very-low-magnification objective lens has a long focal distance and a large radius of lens curvature. Particularly, when performing observation with reflected light, noise such as flare, ghost, or the like which affects original image formation tends to be caused by repeated surface reflection of the lens. In order to solve this, in general, a polarizer is inserted in the reflected light optical system, an analyzer is inserted, in an observation optical system, behind (image side) an objective lens and behind (image side) a half mirror that coaxially introduces the reflected light optical axis into the observation optical axis, and a .lambda./4 plate and a depolarizer are inserted in the distal end (closest to the sample) of the objective lens. When a high-magnification objective lens is employed, the influence of the flare or ghost is small. In this case, the polarizer, the analyzer, the .lambda./4 plate, and the depolarizer need not be used or are better be omitted as they decrease brightness. If the .lambda./4 plate and the depolarizer are mounted to the distal end of the very-low-magnification objective lens mounted to the revolver means, they can be inserted or removed upon the turning operation of the revolver means, thus solving the problem.
(4) Even with this arrangement, since the polarizer and analyzer are left inserted in the optical path, a mechanism is necessary which inserts them in the optical path for very-low-magnification observation and removes them from the optical path for other observation. This leads to a cumbersome operation and complicated arrangement, leading to an increase in cost.
Jpn. Pat. Appln. KOKAI Publication No. 6-109962 discloses a prior art in which an objective lens revolver is turned electrically. To turn the revolver electrically itself is a known technique, and the revolver is not interlocked with a movable portion which is necessary for other microscopic method switching and the like. As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-311342 and No. 63-133115, a technique is known which improves the operability by interlocking insertion/removal of optical elements and the like, required for performing a switching operation among various types of microscopic methods, light control, a stop, a cube, and the like. However, no description is made concerning two types of objective lenses which are inserted in and removed from the optical path in an interlocked manner during magnification switching. Also, no description is made concerning an objective lens for magnification switching, which is inserted in and removed from the optical path with the same drive member as that employed for microscopic method switching. Hence, problems similar to those of Jpn. Pat. Appln. KOKAI Publication No. 9-54253 exist.
From the above reasons, an optical microscope which solves the various problems described above is sought for.